1. Field of the Invention
The present invention relates to a process for preparing ketene acetals by dehydrohalogenation of 2-haloaldehyde acetals.
2. The Prior Art
It is known that ketene acetals can be obtained from the corresponding 2-haloaldehyde acetals by elimination of hydrogen halide with the aid of alkoxides or amine bases. Both Houben-Weyl Vol. 7/4, 4th Ed., Stuttgart 1968, p. 340, and W. J. Bailey, "Ring-Opening Polymerisation," Comprehensive Polymer Science, Vol. 3, Pergamon Press 1989, pp. 283-326, describe hydrogen bromide elimination by means of potassium tert.-butoxide. C.A. 113 (18):153070 discloses the dehydrobromination in the presence of potassium tert.-butoxide and a phase transfer catalyst. The dehydrochlorination of 2-chloromethyl-1,3-dioxolane by means of sodium or potassium in liquid ammonia is described in U.S. Pat. No. 3,431,281. These procedures have the disadvantages that in these methods only the more reactive and also more expensive 2-bromoaldehyde acetals can be used as starting compounds. In addition, the bases which can be used are the relatively expensive (for synthesis on an industrial scale), alkali or alkaline earth metal alkoxides or alkali metals dissolved in liquid ammonia.
WO-A 92/14722 relates to a process for the dehydrohalogenation of halogenated cyclic ketene acetals such as 2-chloromethyl-1,3-dioxepane with alkali or alkaline earth metal hydroxides in the presence of an inert alcohol such as 2-butanol. This procedure has the disadvantage of the low yields (low selectivity and by-products from aqueous work-up) which are obtained in this process after purification, which is complicated into the bargain, of the crude product by fractional distillation in the presence of an organic solvent and an amine.